If you are doing a space based sim (saw it mentioned) perhaps you should be thinking about "simulated distance" as a techique for creating large areas of space.
Its a system my team will use when we get around to our space mmo (the next project after our current one) and I will try describe the system lightly.
Essentially, instead of having a giant great big game area filled mostly with space, you have "hotspots".
1: Each hotspot is a focal point, e.g. a planet, moon, space station, emplacement, etc - you get the idea.
2: Each hotspot is normal or small sized, with manual seamless transitions based on the direction of exit from the "hotspot" - this is done via scripting.
3: Upon exit the hotspot, an
instance is loaded of pre-made empty space with a custom skybox based on the area the player should be in.
The Empty space area:
1: Has a predefined size that has standard seamless transitions, but to another (same area but different instance) empty space instance.
2: On leaving an empty space instance, it is destroyed.
3: Each empty space instance gets a 3d co-ordinates reference in a "virtual" space, like a gigantic rubix cube. A good system to use would be STARNAME-101-32-18 or similar.
Now, to combine this with multiplayer:
1: If a player leaves a hotspot, and enters an instance in the virtual space that is already occupied, instead of building a new instance, an existing instance is entered.
Using scripted placement, distance objects can be created at smaller size in rough direction of where they would be if the space was enoooormous.
For a star system that has 8 planets and 20 moons, the amount of areas required would simply be 29 - 8 planet hotspots, 20 moon hotspots, and a single "empty space" re-usable instance base.
All of the hotspots can also be relatively small as well, thus being more resource friendly.
Another tip is to scale down your player objects to give a sense of more size.
The end result is a highly efficient, very fast, very small universe that ends up appearing to be ridiculously large.
We have only done preliminary experimentation with this proposed system, and the above outlines are working from my memory, but from what I have learned in my short time with hero engine this should definately be doable.
Its just script heavy as opposed to area build heavy

-Jon